1. Field of the Invention
The present invention relates to data recording in a communications system, and, more particularly, to writing information from a channel to a recording medium.
2. Description of the Related Art
Conventional recording systems of the prior art encode data and write the encoded data to a recording medium, such as a magnetic hard drive or an optical recording disc. The encoded data is written to the disc (or other recording medium) as servo sector information by a write head. A servo sector is a prerecorded reference track on the dedicated servo surface of a disc drive. Most magnetic disc drives have many disc surfaces, with each surface having a corresponding magnetic head mounted on an actuator (e.g., a Head Stack Assembly or HSA). All data track positions are compared to their corresponding servo track to determine off-track/on-track position. The cylindrical surface formed by identical track numbers on vertically stacked discs is termed a “cylinder”. At any location under the write head, all tracks under all heads form a cylinder. The cylinder number is one of the three address components required to find a specific address, where the other two address components are head number and sector number. In a wedge-based servo sector system, a certain part of each cylinder contains servo-positioning data (a servo data pattern). Gap spacing (termed a servo “wedge” field or “servo data segment”) between each sector contains a servo data pattern to maintain position on that cylinder.
During fabrication in a factory and before shipment of a hard drive, a servo track writer records the servo data patterns on the recording surface. Writing of a servo pattern is accomplished by the servo track writer during “servo bank write mode” or SBWM. Bank writing writes to multiple surfaces since most drives have many disc surfaces. The total servo track writing time increases dramatically with each new generation of recording drives as capacity increases. Thus, if a servo track writer were to write only one head at a time, the disc drive throughput of the fabrication factory would decrease as the number of tracks and TPI (tracks per inch) increase.
Consequently, it is preferable to enable writing servo data patterns by as many write heads as possible. However, writing is accomplished through a write channel preamplifier (preamp) that is enabled by a signal from a write gate. Increasing the number of heads written at the same time increases the power dissipation of the preamp. Preamps of the prior art may be required to support writing of up to eight channels (i.e., eight heads) at the same time. Increasing the number of heads written at the same time also increases the bandwidth of a read channel amplifier employed to read data from the recording medium during the servo track writing verification process. These competing requirements result in a major challenge to efficiently manage SBWM operation.
Preamps of the prior art generally guarantee writing in SBWM of up to two channels at the same time without relatively high power dissipation. With such a prior art preamp, drive vendors designing 4–8 channel drives cannot write, for example, to all 4–8 heads within one pass of servo bank writing. Typically, servo bank writing is accomplished by writing to the drive in staggered mode. In staggered mode for a 8-head drive, the write head writes track N first on heads 0&1, then on heads 2&3, and then on heads 4&5. After track N is written on all six heads, the write head steps to the next track (N+1) and repeats the process. One drawback to staggered bank write mode is reduced factory throughput since the time required to finish writing 4 or 6 or 8 head drives is doubled or tripled or quadrupled, respectively.
Alternatively, servo data patterns are written with a write gate (WG) opening only during the servo data segment, or servo wedge field, of the disc. “WG opening” refers to the period when the write head is enabled (e.g., by an enabling signal to a logic gate) to allow servo data patterns to be recorded onto the recording medium by the write head. WG opening only during the servo wedge field might dramatically reduce the WG duty cycles required to write an entire disc surface, however, it does not erase an old servo or other data pattern if the disc media was previously recorded on. If an old data pattern is not erased, a servo controller may have difficulty locking onto a newly recorded servo data pattern when the disc drive is in self-test mode. FIG. 1 shows SBWM WG Timing and Duty Cycles for staggered bank mode writing. FIG. 2 shows SBWM WG Timing and Duty Cycles for WG opening for writing only during the servo wedge field.
In FIG. 1, the WG is open within one revolution of disc rotation. In this mode, the WG duty cycle is the timing per RPM divided by the sum of i) the timing of one RPM and ii) the head stepping time. For a 7200 RPM drive application: T1 is the timing of one RPM=60/7200 (=8.33333 ms); T2 is the timing for head stepping (normally around 4 to 5 ms); and the WG duty cycle “WG_Duty_Cycle—0” is approximately T1/(T1+T2)*100%=62.4% to 67.5%.
In FIG. 2, the WG is open only on every servo wedge field. Normally, the time to cover the servo wedge field is about 10% of the time to cover between wedges (i.e., wedge-to-wedge timing), for example, assuming a 7200 RPM drive application. For the example of a 7200 RPM drive, T1=8.333 ms. If N is the number of servo wedges per track, then the wedge-to-wedge timing is approximately T/N. Assuming N=300, then wedge-to-wedge timing (T3) is approximately 27.776 μs and the servo wedge field time (T4) is about 10% of 27.776 μs (i.e., T4≈2.776 μs). Then, within one track, the effective write gate duty cycle (WO Duty Cycle—1′) is T4/T3*100%=10%, and the total WG duty cycle (WG_Duty_Cycle—1) is approximately WG_Duty_Cycle—0*WG_Duty_Cycle—1′=6.24–6.75%